1. Field of the Invention
The present invention relates to a coding apparatus that transforms an m-bit data word into an n-bit code word and performs a coding on the above-mentioned m-bit data word and a method therefor. Also, the invention relates to a recording apparatus that records a recording code sequence obtained through the coding described above and a method therefor. Furthermore, the invention relates to a decoding apparatus that decodes a code sequence and a method therefor.
2. Description of the Related Art
As an optical recording medium where recording/reproduction of a signal is performed though irradiation of light, for example, so-called optical discs such as a CD (Compact Disc), a DVD (Digital Versatile Disc), and a BD (Blu-ray Disc: registered trademark) are widely spread.
In these currently prevalent optical discs, a mark edge recording is carried out in which a recording code is defined by NRZI (Non Return to Zero Inverting) and transformed into an NRZ (Non Return to Zero) code at the time of recording to be then recorded.
Also, for the optical discs, a low frequency content of the recording code is desirably used in connection with a situation that a tracking error signal is obtained from a group, a pit, or the like. That is, this is because a tracking servo channel is located to be lower than a signal channel of the recording code, but when the recording code has a large amount of the low frequency components, the component of the recording code overlaps the tracking error signal, which may degrade a tracking servo characteristic.
In view of the above, in the optical discs in related art, suppression on the low frequency components of the recording code is realized by performing a control so as to lower an absolute value of a DSV (Digital Sum Value) of a recording NRZ code sequence.
For example, an EFM modulation code is used in the CD, where a coding method is adopted in which predetermined merging bits composed of 3 bits that satisfy a limit of a minimum runlength of d=2 and also causes the absolute value of the DSV of the code sequence to be small are selected and inserted between a 14-bit code word and the next code word.
Also, in the DVD, the following DSV control is carried out. By using a modulation code called EFM Plus, with regard to a certain data word, a code word with which the absolute value of the DSV of the code sequence becomes small is selected from a main table and a substitute table to be coded. This modulation code is disclosed, for example, “EFMPlus: THE CODING FORMAT OF THE MULTIMEDIA COMPACT DISC” by Kees A. Schouhamer Immink, IEEE Transaction on Consumer Electronics, Vol. 41, Issue 3, Aug. 1995 or International Publication No. 95/22802.
Also, a modulation code called 17PP is used in the BD. DC control bits are periodically defined in a recording data format of the BD, and a coding is carried out after the DC control bit of “0” or “1” with which the absolute value of the DSV of the code sequence decreases is selected.
Also, for example, for a recording modulation coding method for a magnetic disc recording medium, (2,7) RLL is also used. This (2,7) RLL satisfies runlength constraints of d=2 and also k=7, in which a coding using a variable length table is carried out. The coding rate becomes ½.
To be more specific, in (2,7) RLL, the coding on input data is carried out on the basis of the following rule.
Input data→Code
11→1000
10→0100
000→000100
010→100100
011→001000
0011→00001000
0010→00100100
According to such transform rules of (2,7) RLL, for example, input data composed of “110110011” is transformed into a code sequence of “100000100000001000” on the basis of “11”→“1000”, “011”→“001000”, and “0011”→“00001000”.
On the other hand, in contrast with the currently prevalent optical discs such as the CD, the DVD, and the BD, the applicant of the present invention previously proposes a so-called bulk recording-type (simply also referred to as bulk-type) optical disc as a next generation optical disc as described, for example, in Japanese Unexamined Patent Application Publication No. 2008-135144 or Japanese Unexamined Patent Application Publication No. 2008-176902.
Here, the bulk recording refers to a technology for realizing a larger recording capacity by irradiating an optical recording medium (a bulk-type recording medium 100) having at least a cover layer 101 and a bulk layer (recording layer) 102 with laser light while sequentially changing a focal position and performing a multilayer recording in the bulk layer 102 as illustrated, for example, in FIG. 18.
With regard to the above-mentioned bulk recording, Japanese Unexamined Patent Application Publication No. 2008-135144 described above discloses a recording technology that is so-called micro hologram system. According to the micro hologram system, for a recording material of the bulk layer 102, so-called hologram recording material is used. For the hologram recording material, for example, light-cured type photopolymer or the like is widely used.
The micro hologram system is roughly classified into a positive-type micro hologram system and a negative-type micro hologram system.
The positive-type micro hologram system relates to a technique for gathering two opposed light fluxes (a light flux A and a light flux B) at a same position to form a minute interference pattern (hologram) and setting this as a recording mark.
Also, on the basis of the opposite point of view from the positive-type micro hologram system, the negative-type micro hologram system relates to a technique for deleting a previously formed interference pattern through laser light irradiation and setting the deleted part as a recording mark. In this negative-type micro hologram system a processing of forming the interference pattern in the bulk layer is performed in advance as an initialization processing.
Also, the applicant of the present invention also proposes, for example, a recording method of forming void (porosity, hole) as a recording mark which is disclosed in Japanese Unexamined Patent Application Publication No. 2008-176902 as a method for the bulk recording that is different from the micro hologram system.
A void recording system relates to a method of irradiating the bulk layer 102 made of a recording material such as light-cured type photopolymer, for example, with laser light at relatively high power and recording the void in the bulk layer 102. As described in Japanese Unexamined Patent Application Publication No. 2008-176902, the thus formed void section becomes a section having a different refractive index from other sections in the bulk layer 102, and a reflection factor is increased at a border section thereof. Therefore, the above-mentioned void section functions as the recording mark, and information recording based on the formation of the void section is accordingly realized.
According to the above-mentioned void recording system, the hologram is not formed, and it suffices that the light irradiation is carried out from one side at the time of the recording. That is, in the case of the above-mentioned positive-type micro hologram system, it suffices that the recording mark is not formed by gathering the two light fluxes at the same position.
Also, in comparison with the negative-type micro hologram system, a merit exists that the initialization processing is not carried out.
It should be noted that Japanese Unexamined Patent Application Publication No. 2008-176902 illustrates an example in which precure light irradiation before recording is performed when the void recording is carried out, but the void recording can be carried out even when such precure light irradiation is omitted.
Incidentally, with regard to the bulk recording-type (simply also referred to as bulk-type) optical disc recording medium for which the above-mentioned various recording techniques are proposed, a recording layer (bulk layer) of the above-mentioned bulk-type optical disc recording medium does not have an explicit multilayer structure in a sense, for example, that a plurality of reflection films are formed. That is, the bulk layer 102 is not provided with a reflection film and a guide groove for each recording layer which are provided to a normal multilayer disc.
Therefore, in the case of the structure of the bulk-type recording medium 100 illustrated in the aforementioned FIG. 18 without change, at the time of recording when the mark is not formed, focus servo and tracking servo are not carried out.
For this reason, in the actuality, the bulk-type recording medium 100 is provided with a reflection surface (reference surface) that becomes a reference having a guide groove as illustrated in the following FIG. 19.
To be more specific, for example, a guide groove (position guiding element) is formed on a lower surface side of the cover layer 101 in a spiral or concentric manner through formation of a pit or a groove where a selective reflection film 103 is formed. Then, the bulk layer 102 is layered via an adhesive material such as UV curing resin, for example, functioning as an intermediate layer 104 in the drawing on a lower layer side of the cover layer 101 on which the selective reflection film 103 is thus formed.
Here, through the above-mentioned formation of the guide groove by the pit or the groove, for example, recording of absolute positional information (address information) such as radial positional information or rotation angle information is carried out. In the following description, the surface on which the guide groove is formed and the recording of the absolute positional information is carried out (in this case, the formation surface of the selective reflection film 103 described above) is referred to as “reference surface Ref”.
On the basis of the above-mentioned medium structure, the bulk-type recording medium 100 is irradiated with not only laser light for recording (or reproducing) the mark as illustrated in the drawing (hereinafter, which will be also referred to as recording reproduction laser light or simply recording reproduction light) but also laser light for servo (simply also referred to as servo light) as laser light for positional control via respective common objective lenses.
At this time, if the above-mentioned servo laser light reaches the bulk layer 102, an adverse affect may occur in the mark recording in the bulk layer 102. For this reason, up to now, in the bulk recording system, while laser light having a different waveband from the recording reproduction light is used as the above-mentioned servo laser light, the selective reflection film 103 having a wavelength selectivity in which the servo laser light reflects and the recording reproduction laser light transmits is provided as the reflection film formed on the reference surface Ref.
On the basis of the above-mentioned assumption, a description will be given of an operation at the time of the mark recording on the bulk-type recording medium 100. First, when the multilayer recording is carried out on the bulk layer 102 the guide groove or the reflection film is not formed, at which positions are previously set as layer positions for recording the mark in a depth direction in the bulk layer 102. In the drawing, a case is exemplified in which total five information recording layer positions including a first information recording layer position L1 to a fifth information recording layer position L5 are set as the layer positions where the mark is formed in the bulk layer 102 (mark formation layer position: also referred to as information recording layer position). As illustrated in the drawing, the first information recording layer position L1 is the information recording layer position L set on the uppermost layer, and subsequently, in the order of L2→L3→L4→L5, the information recording layer position L is set on the lower layer side.
At the time of recording when the mark is not formed, the focus servo and the tracking servo are not carried out while the respective layer positions in the bulk layer 102 are set as targets on the basis of the reflection light of the recording reproduction laser light. Therefore, the focus servo control and the tracking servo control on the objective lens at the time of recording are carried out on the basis of the reflection light of the servo laser light so that a spot position of the servo laser light follows the guide groove on the reference surface Ref.
It should be noted however that the above-mentioned recording reproduction laser light is caused to reach the bulk layer 102 formed on the lower layer side of the reference surface Ref for the mark recording, and also selection of the focal positions in the bulk layer 102 is set to be available. For this reason, an optical system in this case is provided with a focus mechanism for the recording reproduction light (expander) for adjusting a focal position of the recording reproduction laser light separately from a focus mechanism of the objective lens.
In other words, with the thus provided expander, by changing a collimation of the recording reproduction laser light that enters the objective lens, the focal position of the recording reproduction laser light is adjusted independently from the servo laser light.
A position in a tracking direction of the recording reproduction laser light is automatically controlled to be at a position immediately below the guide groove on the reference surface Ref by the tracking serve of the objective lens using the above-mentioned servo laser light.
It should be noted that when the bulk-type recording medium 100 where the mark recording is already carried out is reproduced, unlike the recording time, the position of the objective lens is not controlled on the basis of the reflection light of the servo laser light. That is, at the time of reproduction, while a mark sequence formed on the information recording layer position L that is a reproduction target (which is also referred to as information recording layer L or the mark formation layer L at the time of reproduction) is set as a target, the focus servo control and the tracking servo control on the objective lens is carried out on the basis of the reflection light of the recording reproduction laser light.